As an illumination device including light-emitting diodes (LEDs) to be lit, an LED illumination device for detecting a failure of the LEDs has been known (e.g., see JP 2007-305929 A which is hereinafter referred to as “Document 1”).
In the LED illumination device described in Document 1, field effect transistors (FETs) are inserted in parallel to all the LEDs. The LED illumination device described in Document 1 detects a failed LED of the LEDs and turns on the FET, so that a current flows while bypassing the failed LED.
In a conventional illumination device as described in Document 1, when a light-emitting element such as an LED fails so as to become an open circuit, a current no longer flows through the light-emitting element, and therefore, a light emission defect of the light-emitting element is detectable.
The conventional illumination device, however, has a problem that an oscillation defect of the light-emitting element is not detectable. The oscillation defect is that a light-emitting element emits no light although a current continues to flow through the light-emitting element. The “light emission defect of the light-emitting element” includes not only a defect that a current does not flow through the light-emitting element and thus the light-emitting element emits no light but also the oscillation defect of the light-emitting element. The “oscillation defect of the light-emitting element” is a defect that a current continues to flow through a light-emitting element, but the light-emitting element emits no light due to a problem occurring in an oscillation operation of the light-emitting element. As a property of the light-emitting element, the oscillation defect may occur.
To solve the problem, the number of light-emitting elements connected in series to each other is increased, so that even when the oscillation defect of the light-emitting element occurs, the influence of the oscillation defect over the quantity of light emitted from the plurality of light-emitting elements can be reduced. However, increasing the number of light-emitting elements leads to a new problem, that is, increased cost.